Electrodes on a semiconductor device and external connection terminals are connected using a bonding wire for a semiconductor (hereinafter may also be referred to as a “bonding wire”). In order to bond the bonding wire to electrodes on a silicon chip as the semiconductor element, there is performed ball bonding featuring a thermal compressive bonding technique with the aid of ultrasound. On the other hand, when connecting the bonding wire to the external connection terminals such as a lead and a land, there is generally performed so-called 2nd bonding that bonds the bonding wire directly to the electrodes without forming a ball.
For a material of the bonding wire, Au bonding wires (gold bonding wires) with a wire diameter of about 15 to 50 μm and a material of Au (gold) with high purity 4N (4-Nine, a purity of 99.99% by mass or more) have been conventionally and mainly used.
With resource prices escalating in recent years, there has also been a steep rise in the price of gold as a raw material of the Au bonding wire, and Cu (copper) has been considered as an alternative to Au as a low-cost material for a wire. However, Cu is more susceptible to oxidation than Au, and a simple copper bonding wire is difficult to be stored for a long period of time and is not favorable in 2nd bonding characteristics. In addition, when a ball is formed at the tip of such a simple Cu bonding wire, a reductive atmosphere is required so that the ball will not be oxidized.
Given these circumstances, in order to solve the problem of oxidation of Cu bonding wire, there has been proposed a Cu bonding wire in which a surface of Cu wire is coated with a noble metal. Patent Literature 1 discloses a Cu bonding wire in which a surface of a high-purity Cu fine wire is coated with a noble metal such as Au and Pd.
The Cu bonding wire in which a surface of Cu wire is coated with a noble metal is excellent in long-term storage and 2nd bonding characteristics of the wire, because oxidation of the Cu bonding wire is suppressed. In particular, regarding a Cu bonding wire in which a surface of Cu wire is coated with Pd, there can be significantly remedied concerns about oxidation of a ball when forming the ball at a tip of the wire, and a spherical ball can be formed by simply surrounding the ball with a nitrogen atmosphere prepared using a pure nitrogen gas, without using a hazardous gas such as hydrogen.